Embedded DRAM for extremely thin semiconductor-on-insulator

ABSTRACT

A node dielectric and a conductive trench fill region filling a deep trench are recessed to a depth that is substantially coplanar with a top surface of a semiconductor-on-insulator (SOI) layer. A shallow trench isolation portion is formed on one side of an upper portion of the deep trench, while the other side of the upper portion of the deep trench provides an exposed surface of a semiconductor material of the conductive fill region. A selective epitaxy process is performed to deposit a raised source region and a raised strap region. The raised source region is formed directly on a planar source region within the SOI layer, and the raised strap region is formed directly on the conductive fill region. The raised strap region contacts the raised source region to provide an electrically conductive path between the planar source region and the conductive fill region.

BACKGROUND

The present invention relates to an embedded dynamic random accessmemory (eDRAM) structure for an extremely thinsemiconductor-on-insulator (SOI) substrates and methods of manufacturingthe same.

Embedded dynamic random access memory (eDRAM) is a dynamic random accessmemory (DRAM) embedded in a logic circuit to function as a high densitycache memory. The eDRAM provides comparable access time as static randomaccess memory (SRAM) at a smaller device area per cell. Typically, eDRAMarrays are employed as a level-2 (L2) cache or level-3 (L3) cache in aprocessor to provide a high density memory in a processor core. Due tohigh performance and a compact size, eDRAM has become one of the mostefficient means for continued performance of semiconductor logiccircuits requiring embedded memory including processors andsystem-on-chip (SoC) devices.

Semiconductor-on-insulator (SOI) substrates are employed in thesemiconductor industry for performance benefits due to reducedcapacitive coupling between semiconductor devices and the bulk portionof the substrate provided by a buried insulator layer. High performancelogic chips are frequently manufactured on an SOI substrate to provideenhanced performance over devices having comparable dimensions andmanufactured on a bulk substrate. Extremely thinsemiconductor-on-insulator (ETSOI) substrate refers to a substrateincluding an extremely thin semiconductor-on-insulator layer, whichtypically has a thickness less than 100 nm, and more typically less than50 nm.

A conventional eDRAM cell employing a deep trench capacitor provides anelectrical connection between the deep trench capacitor and a source ofan access transistor via a conductive buried strap, which is “buried”underneath a portion of a shallow trench isolation structure. In thecase of an ETSOI substrate, the limited thickness of the ETSOI layerpresents a unique challenge. The ETSOI layer is too thin to accommodatereliable manufacturing of a buried strap underneath a portion of ashallow isolation trench structure. The thickness of the ETSOI layerlimits the vertical range of contact between the buried strap and thesource of the access transistor. Further, even moderate variations inthe recess depth corresponding to a bottom surface of the buried strapor the recess depth corresponding to the top surface of the buried strapcan lead to significant variations in the resistance of the electricalpath between the source and the inner node of the deep trench.Nonetheless, controlled and limited resistance between the source of theaccess transistor and the inner node of the deep trench is necessary toprovide high performance from an eDRAM cell.

BRIEF SUMMARY

A node dielectric and a conductive trench fill region filling a deeptrench are recessed to a depth that is substantially coplanar with a topsurface of a semiconductor-on-insulator (SOI) layer. A shallow trenchisolation portion is formed on one side of an upper portion of the deeptrench, while the other side of the upper portion of the deep trenchprovides an exposed surface of a semiconductor material of theconductive fill region. An access transistor is formed in an activeregion laterally contacting an outer sidewall of the node dielectric. Aselective epitaxy process is performed to deposit a raised source regionand a raised strap region. The raised source region is formed directlyon a planar source region within the SOI layer, and the raised strapregion is formed directly on the conductive fill region. The raisedstrap region contacts the raised source region to provide anelectrically conductive path between the planar source region and theconductive fill region.

According to an aspect of the present invention, a semiconductorstructure is provided, which includes: a deep trench located in asemiconductor-on-insulator (SOI) substrate that includes a topsemiconductor layer containing a planar source region of an accesstransistor; a conductive trench fill region located in the deep trenchand contacting an inner sidewall of the node dielectric; a raised sourceregion located above the top semiconductor layer and contacting an uppersurface of the planar source region; and a raised strap regioncontacting the raised source region and an upper surface of theconductive trench fill region.

According to another aspect of the present invention, a method offorming a semiconductor structure is provided, which includes: forming adeep trench in a semiconductor-on-insulator (SOI) substrate; forming aconductive trench fill region in the deep trench, wherein an activeregion in a top semiconductor layer of the SOI substrate does notcontact the conductive trench fill region; and depositing asemiconductor material by selective epitaxy on an exposed surface of theactive region and an exposed surface of the conductive trench fillregion, wherein a raised source region is formed directly on the exposedsurface of the active region and a raised strap region is formeddirectly on the exposed surface of the conductive trench fill region,and the raised source region contacts the raised strap region.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of a first exemplarysemiconductor structure after formation of a pad layer on asemiconductor-on-insulator (SOI) substrate, application of a firstphotoresist, and lithographic patterning of the first photoresistaccording to a first embodiment of the present invention.

FIG. 2 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of a deep trench, a buriedplate, a node dielectric, a conductive trench fill region, and recessingof the conductive trench fill region and the node dielectric accordingto the first embodiment of the present invention.

FIG. 3 is a vertical cross-sectional view of the first exemplarysemiconductor structure after application of a second masking layer andlithographic patterning of the second masking layer according to thefirst embodiment of the present invention.

FIG. 4 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of a prototype shallow trenchisolation structures according to the first embodiment of the presentinvention.

FIG. 5 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of gate stacks and variousplanar source and drain regions according to the first embodiment of thepresent invention.

FIG. 6 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of raised source regions, raisedstrap regions, and raised drain regions according to the firstembodiment of the present invention.

FIG. 7 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of various metal semiconductoralloy portions according to the first embodiment of the presentinvention.

FIG. 8 is a vertical cross-sectional view of the first exemplarysemiconductor structure after formation of a dielectric layer andcontact via structures embedded therein according to the firstembodiment of the present invention.

FIG. 9 is a magnified view of a portion of the first exemplarysemiconductor structure of FIG. 8.

FIG. 10 is a vertical cross-sectional view of a second exemplarysemiconductor structure after selective epitaxy of raised sourceregions, raised strap regions, and raised drain regions according to asecond embodiment of the present invention.

FIG. 11 is a vertical cross-sectional view of a third exemplarysemiconductor structure according to the third embodiment of the presentinvention.

FIG. 12 is a vertical cross-sectional view of a fourth exemplarysemiconductor structure according to a fourth embodiment of the presentinvention.

DETAILED DESCRIPTION

As stated above, the present invention relates to an embedded dynamicrandom access memory (eDRAM) structure for an extremely thinsemiconductor-on-insulator (SOI) substrates and methods of manufacturingthe same, which are now described in detail with accompanying figures.It is noted that like and corresponding elements mentioned herein andillustrated in the drawings are referred to by like reference numerals.

A “deep trench” herein refers to a trench that extends from a topsurface of a top semiconductor layer to a depth below a bottom surfaceof a buried insulator layer in a semiconductor-on-insulator (SOI)substrate.

Referring to FIG. 1, a first exemplary semiconductor structure accordingto a first embodiment of the present invention includes asemiconductor-on-insulator (SOI) substrate 8, a pad layer 40, and afirst masking layer 17. The first exemplary semiconductor structurecomprises an embedded dynamic random access memory (eDRAM) region E, inwhich eDRAM cells are to be subsequently formed, and a logic region L,in which at least another semiconductor device is to be subsequentlyformed.

The SOI substrate 8 contains a handle substrate 10, a buried insulatorlayer 20, and a top semiconductor layer 30. The handle substrate 10comprises a semiconductor material such as silicon. Typically, thehandle substrate 10 comprises a single crystalline semiconductormaterial. The handle substrate 10 can be undoped or have a p-type dopingor an n-type doping. If the handle substrate includes a dopedsemiconductor material, the handle substrate 10 can be doped at a dopantconcentration from 1.0×10¹³/cm³ to 3.0×10¹⁷/cm³.

The buried insulator layer 20 comprises a dielectric material such assilicon oxide and/or silicon nitride. For example and in one embodiment,the buried insulator layer 20 can comprise thermal silicon oxide. Thethickness of the buried insulator layer 20 can be from 5 nm to 1000 nm,and typically from 100 nm to 200 nm. The buried insulator layer maycomprises multiple dielectric layer, e.g., silicon oxide and siliconnitride.

The top semiconductor layer 30 comprises a semiconductor material. Thethickness of the top semiconductor layer 30 can be from 2 nm to 100 nm,and typically from 5 nm to 20 nm, although lesser and greaterthicknesses can also be employed. If the top semiconductor layer 30 hasa thickness less than 100 nm, the SOI substrate is referred to as anextremely thin semiconductor-on-insulator layer. In one embodiment, thetop semiconductor layer 30 comprises a single crystalline semiconductormaterial. The semiconductor material of the top semiconductor layer 30can be selected from, but is not limited to, silicon, germanium,silicon-germanium alloy, silicon carbon alloy, silicon-germanium-carbonalloy, gallium arsenide, indium arsenide, indium phosphide, III-Vcompound semiconductor materials, II-VI compound semiconductormaterials, organic semiconductor materials, and other compoundsemiconductor materials. The top semiconductor layer 30 can have abuilt-in biaxial stress in a horizontal plane, i.e., in the planeparallel to the interface between the buried insulator layer 20 and thetop semiconductor layer 30.

The conductivity type of the portion of the top semiconductor layer 30within the eDRAM region E is herein referred to as a first conductivitytype, which can be p-type or n-type. The portion of the topsemiconductor layer 30 within the logic region L can includesub-portions having a doping of the first conductivity type and/orsub-portions having a doping of a second conductivity type, which is theopposite of the first conductivity type. For example, if the firstconductivity type is p-type, the second conductivity type is n-type, andvice versa.

The pad layer 40 is formed directly on the top semiconductor layer 30 bya blanket deposition employing, for example, chemical vapor deposition(CVD). In one embodiment, the pad layer 40 comprises a dielectricmaterial such as a dielectric nitride or a dielectric oxide. The padlayer 40 can comprise a stack of multiple dielectric layers or acombination of at least one dielectric layer and at least onesemiconductor layer comprising silicon or a silicon containing alloy.For example and in one embodiment, the pad layer 40 is a silicon nitridelayer. In another example, the pad layer 40 comprises a stack of a thinsilicon oxide layer formed directly on the top semiconductor layer 30, athicker silicon nitride layer on the thin silicon oxide layer, and athicker silicon oxide layer on the silicon nitride layer. The thicknessof the pad layer 40 can be from 40 nm to 1,000 nm, and typically from 50nm to 150 nm. In one embodiment, the pad layer 40 can be formed directlyon the top semiconductor layer 30 by chemical vapor deposition (CVD).

A first masking layer 17 is applied over the pad layer 40 andlithographically patterned to form openings in the first masking layer17, which is herein referred to as deep trench openings DTO, in theeDRAM region E.

Referring to FIG. 2, the pattern in the first masking layer 17 istransferred into the pad layer 40, the top semiconductor layer 30, theburied insulator layer 20, and the handle substrate 10 to form deeptrenches. While a pair of deep trenches is shown in the drawings forillustrative purposes, the present invention can be practiced in otherconfigurations including any number of deep trenches in the eDRAM regionE. In one embodiment, the masking layer 17 is a photoresist layer (softmasking layer). In another embodiment, the masking layer 17 compriseshardmask (e.g., silicon oxide).

Specifically, the pattern including the deep trench opening DTO in thefirst masking layer 17 is transferred into the pad layer 40, the topsemiconductor layer 30, the buried insulator layer 20, and an upperportion of the handle substrate 10 to form deep trenches by methodsknown in the art such as reactive ion etch (RIE). The depth of the deeptrenches, as measured from a top surface of the SOI substrate 8 tobottom surfaces of the deep trenches, can be from 0.8 mm to 20 μm, andtypically from 2 μm to 8 μm. A portion of the pad layer 40 can beconsumed during the formation of the deep trenches or be removed afterthe formation of the deep trenches.

At least one buried plate 6 is formed outside the sidewalls of the deeptrenches. The conductivity type of the dopants can be the same or theopposite of the doping of the remaining portions of the handle substrate10 if the first handle substrate 10 includes a doped semiconductormaterial. The buried plate 6 can be formed by diffusing dopants into thehandle substrate 10 from within the deep trenches. For example, the atleast one buried plate 6 can be formed in the handle substrate 10 bydepositing a dopant containing material layer (not shown) such asarsenosilicate glass (ASG) and outdiffusion dopants into the portion ofthe handle substrate 10 that directly abuts sidewalls of the deeptrenches beneath a bottom surface of the buried insulator layer 20. Thedopant containing material layer is subsequently removed. The at leastone buried plate 6 can be formed by any other alternative method,including but not limited to, ion implantation, gas phase doping, plasmadoping, plasma immersion ion implantation, infusion doping, and liquidphase doping. The at least one buried plate 6 can be a plurality ofburied plates. Each buried plate 6 can be formed around a single deeptrench, or can be formed around a pair of deep trenches. Alternatively,at least a portion of the handle substrate 10 can be doped duringfabrication of the SOI substrate. For example, a doped layer can beformed (e.g., by doped epitaxial growth) in the handle substrate 10before it is bonded to the top semiconductor layer 30 through the buriedinsulator layer 20.

A node dielectric 50 is formed directly on sidewalls of each deeptrench. The node dielectric 50 can comprise silicon oxide, siliconnitride, silicon oxynitride, high-k material, or any combination ofthese materials. Examples of high-k material include, but are notlimited to, metal oxides such as hafnium oxide, hafnium silicon oxide,hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide,zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride,tantalum oxide, titanium oxide, barium strontium titanium oxide, bariumtitanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide,lead scandium tantalum oxide, and lead zinc niobate, and any combinationof these materials. Forming the node dielectric 50, depending on thenode dielectric material, can include performing a thermal oxidation,chemical oxidation, thermal nitridation, atomic layer deposition (ALD),chemical vapor deposition (CVD), low-pressure chemical vapor deposition(LPCVD), and/or any other suitable methods. In one embodiment, the nodedielectric 50 comprises oxynitride formed by thermal nitridationfollowed by LPCVD and thermal oxidation.

Subsequently, a conductive trench fill material is deposited, forexample, by low pressure chemical vapor deposition (LPCVD). Theconductive trench fill material can be, for example, doped polysilicon.Alternative conductive trench fill materials include, but are notlimited to, germanium, a silicon germanium alloy, a metal (e.g.,tungsten), a conducting metallic compound material (e.g., titaniumnitride), and conductive carbon. Using any suitable combination of thoseconductive materials is also explicitly contemplated herein. Each deeptrench is filled with a doped semiconductor material or a combination ofa metallic material and a doped semiconductor material such that theupper portion of the deep trench above a top surface of the buriedinsulator layer 20 is composed of a doped semiconductor material. Thus,at least an upper portion of the conductive trench fill material abovethe top surface of the buried insulator layer 20 is composed of a dopedsemiconductor material such as doped polysilicon, doped germanium, adoped silicon germanium alloy, a doped silicon carbon alloy, a dopedsilicon germanium carbon alloy, a doped III-V compound semiconductor, adoped II-VI compound semiconductor, or a combination thereof. In oneembodiment, the entirety of the conductive trench fill material is adoped semiconductor material.

The portion of the conductive trench fill material above the pad layer40 is removed by planarization, which can employ chemical mechanicalpolishing (CMP), a dry etch, and/or a wet etch. For example, the trenchfill material can be planarized by chemical mechanical polishing (CMP)employing the pad layer 40 as a stopping layer. The portion of theconductive trench fill material within each trench constitutes aconductive trench fill region 60. Each conductive trench fill region 60is subsequently recessed from a top surface of the pad layer 40 to arecess depth r, for example, by a recess etch. The recess etch can beeffected by a reactive ion etch that removes the conductive trench fillmaterial selective to the pad layer 40. The recess depth r substantiallycoincides, i.e., coincides within the controllability of the recessprocess step, with the interface between the top semiconductor layer 30and the pad layer 40. The portion of the node dielectric 50 above therecess depth r of the conductive trench fill region 60 is removed atthis point, for example, by a wet etch.

The conductivity type of the doped semiconductor material of theconductive trench fill region 60 is the second conductivity type. Theportion of the top semiconductor layer 30 within the eDRAM region E havea doping of the first conductivity type, which is the opposite type ofthe second conductivity type, at this point. In one embodiment, thesecond conductivity type is the same as the conductivity type of thedoping of the buried plate 6. In another embodiment, the secondconductivity type is the opposite of the conductivity type of the dopingof the buried plate 6.

After recessing the conductive trench fill regions 60, each nodedielectric 50 extends from the bottom surface of the corresponding deeptrench to a vertical height above the topmost surface of the buriedinsulator layer 20, i.e., above the interface between the buriedinsulator layer 20 and the top semiconductor layer 30. Thus, the topmostsurface of the node dielectric 50 and the topmost surface of theconductive trench fill region 60 within each deep trench are locatedabove the topmost surface of a buried insulator layer 20 after therecessing.

Referring to FIG. 3, a second masking layer 27 is applied over the padlayer 40 and the conductive trench fill regions 60. The second maskinglayer 27 can be a soft masking layer (e.g., photoresist) or a hardmasklayer (e.g., silicon oxide). At least one first opening O1 and at leastone second opening O2 are formed in the second masking layer 90 bylithographic methods. Specifically, the second masking layer 27 islithographically patterned to form a first opening O1 over each pair ofdeep trenches, or alternately over each deep trench if deep trenches arelaterally separated without forming adjacent pairs, in the eDRAM regionE. An edge of the patterned second masking layer 27 around a firstopening O1 overlies a middle portion of each conductive trench fillregion 60. Further, the pattern in the second masking layer 27 includesthe at least one second opening O2, which is located outside the area ofthe deep trenches. Each second opening O2 can be located at a boundarybetween the eDRAM region E and the logic region L or within the logicregion L.

Referring to FIG. 4, the pattern of the at least one first opening O1and the at least one second opening O2 is transferred into the topsemiconductor layer 30 and the conductive trench fill region 60, forexample, by at least one anisotropic etch. The at least one anisotropicetch can include a first anisotropic etch that removes exposed portionsof the pad layer 40. In one embodiment, the first etch can be selectiveto the material of the conductive trench fill region 60. The at leastone anisotropic etch can include a second anisotropic etch, which can beemployed after the first anisotropic etch to etch the exposed portionsof the top semiconductor layer 30 and the exposed portions of theconductive trench fill region 60 within the at least one first openingO1 and the at least one second opening O2. The portion of the nodedielectric 50 between removed portions of the top semiconductor layer 30and the removed portions of the conductive trench fill regions 60 can beremoved during the second anisotropic etch or during a subsequent dryetch or a subsequent wet etch. The second etch can be end-pointed upondetection of the top surface of the buried insulation layer 20 with, orwithout a built-in overetch. Cavities are formed within upper portionsof the SOI substrate 8, which are herein referred to as shallowtrenches, i.e., trenches that do not extend below the bottom surface ofthe buried insulator layer 20. The shallow trenches can include portionsthat extend deeper than exposed portions of the top surface of theburied insulator layer 20. The second masking layer 27 can besubsequently removed selective to the pad layer 40.

A dielectric material such as silicon oxide is deposited within eachshallow trench and is subsequently planarized to form various prototypeshallow trench isolation structures, which include at least one firstprototype shallow trench isolation structure 100A′ and at least onesecond prototype shallow trench isolation structure 100B′ correspondingto the area of the at least one first opening O1 in FIG. 3 and the areaof the at least one second opening O2 in FIG. 3, respectively. Afterplanarization, the top surfaces of the at least one first prototypeshallow trench isolation structure 100A′ and the at least one secondprototype shallow trench isolation structure 100B′ are coplanar with thetop surface of the pad layer 40.

Referring to FIG. 5, the prototype shallow trench isolation structures(100A′, 100B′) are recessed to a depth so that top surfaces of theconductive trench fill regions 60 are exposed after the recess etch. Thepad layer 40 is removed selective to the top semiconductor layer 30, theconductive trench fill regions 60, and the remaining portions of theprototype shallow trench isolation structures (100A′, 100B′). The padlayer 40 can be removed, for example, by a wet etch. If the pad layer 40comprises silicon nitride, the wet etch can employ hot phosphoric acid.Each remaining portion of a first prototype shallow trench isolationstructure 100A′ is herein referred to as a first shallow trenchisolation structure 100A. Each remaining portion of a second prototypeshallow trench isolation structure 100B′ is herein referred to as asecond shallow trench isolation structure 100B. The top surfaces of theconductive trench fill regions 60, the first shallow trench isolationstructure(s) 100A, and the second shallow trench isolation structure(s)100B can be substantially coplanar with one another, i.e., coplanarwithin the limitations of the processes employed to form the varioussurfaces.

Because the topmost surfaces of the conductive trench fill regions 60are exposed after the recess etch, the lateral extent of each firstshallow trench isolation structure 100A does not encompass the entiretyof a deep trench area. Consequently, the topmost surface of eachconductive trench fill region 60 does not underlie the first shallowtrench isolation structure 100A located thereupon. The topmost surfaceof each conductive fill region 60 is exposed at this step.

The remaining portions of the top semiconductor layer 30 constituteactive regions, i.e., semiconductor regions on which semiconductordevices are formed. Transistors are subsequently formed on the activeregions. For example, the transistors can include at least one firsttransistor T1 located in the eDRAM region E and at least one secondtransistor T2 located in the logic region L. Each of the at least onefirst transistor T1 is an access transistor for a capacitor C, whichincludes a buried plate 6 as an outer electrode, a node dielectric 50,and a conductive trench fill regions 60 as an inner electrode. Theactive regions do not contact any conductive trench fill region 60. Eachactive region within the eDRAM region E is laterally spaced from anadjacent conductive trench fill region 60 by a node dielectric 50.

Gate stacks are formed on the active regions employing methods known inthe art. For example, at least one first gate stack is formed on theactive regions within the eDRAM region E, and at least one second gatestack is formed on the active regions within the logic region L. Eachfirst gate stack includes a first gate dielectric 32, a first gateconductor 34, and optionally a first dielectric gate cap 38. A firstdielectric gate spacer 36 is typically formed on sidewalls of each firstgate stack. Each second gate stack includes a second gate dielectric 42,a second gate conductor 44, and optionally a second dielectric gate cap48. A second dielectric gate spacer 46 is typically formed on sidewallsof each second gate stack. Each portion of a first gate stack (32, 34)over a first shallow trench isolation structure 100A is referred to aspassing gate stack, and each portion of a first gate stack (32, 34) overan active region is referred to as an active gate stack. A gate stackstructure typically includes multiple active gate stacks and multiplepassive gate stacks that alternate over active regions and first shallowtrench isolation structures 100A. At this step, the active regionswithin the eDRAM region E have a doping of the first conductivity type.

Subsequently, dopants of the second conductivity type are introduced,for example, by conventional ion implantation or plasma doping, intoexposed portions of the top semiconductor layer 30 within the eDRAMregion E to form first planar source regions 33 and first planar drainregions 37. The first gate stacks (32, 34) are employed as a mask duringthe conventional ion implantation or the plasma doping. Additionalimplantation masks may be implanted to protect portions of active areaswithin the logic region L from implantation of ions. The conventionalion implantation and the plasma doping are herein collective referred toas ion implantation. Alternatively, the formation of the source regions33 and the drain regions 37 can be omitted at this step as they can beformed in the subsequent raised source/drain process by forming dopedRSD and then outdiffusing dopants in RSD into the semiconductor layer 30to form the source/drain regions.

The portions of the top semiconductor layer 30 underlying the first gatestacks (32, 34) within the eDRAM region E constitute a first body region35 of one of the at least one first transistor T1. The first bodyregions 35 have a doping of the first conductivity type, and the firstplanar source regions 33 and the first planar drain regions 37 have adoping of the second conductivity type. The first planar source regions33 and the first planar drain regions 37 are located within the activeregions of the eDRAM region E between the topmost surface of the buriedinsulator layer 20 and the topmost surface of the SOI substrate 8. Eachfirst transistor T1 includes a first gate dielectric 32, a first gateelectrode 34, a first body region 35, a first planar source region 33,and a first planar drain region 37. Each node dielectric 50 is locatedin a deep trench and contacts a sidewall of a planar source region 33.Each conductive fill region 60 does not contact any first planar sourceregion 33, and is laterally spaced from the most adjacent first planarsource region 33 by a node dielectric 50. An outer sidewall of eachfirst shallow trench isolation structure 100A is laterally spaced fromthe most adjacent first planar source region 33 by a portion of a nodedielectric 50 and an upper portion of the conductive trench fill region60.

Additional dopants can be introduced by ion implantation into exposedportions of the top semiconductor layer 30 within the logic region L toform second planar source regions 43 and second planar drain regions 47.The second gate stacks (42, 44) are employed as a mask during the ionimplantation. Additional implantation masks may be implanted to protectthe eDRAM region E and/or other active areas within the logic region Lfrom implantation of ions.

The portions of the top semiconductor layer 30 underlying the secondgate stacks (42, 44) within the logic region L constitute a second bodyregion 45 of one of the at least one second transistor T2. Within eachsecond transistor T2, the second body region 45 has a doping of theopposite conductivity type than the second planar source region 43 andthe second planar drain region 47. The logic region L may contain p-typetransistors and n-type transistors to provide complementarymetal-oxide-semiconductor (CMOS) devices.

Referring to FIG. 6, various raised source regions, various raised strapregions, and various raised drain regions are formed by selectiveepitaxy of a semiconductor material. Specifically, a semiconductormaterial is deposited by selective epitaxy on exposed surfaces of theactive regions and exposed surfaces of the conductive trench fillregions. During the selective epitaxy, the semiconductor material isdeposited only on semiconductor surfaces, but is not deposited ondielectric surfaces such as the surfaces of the first and seconddielectric gate caps (38, 48), the first and second dielectric gatespacers (36, 46) and the first and second shallow trench isolationstructures (100A, 100B). A first raised source region 53 is formeddirectly on each exposed surface of the first planar source regions 33.A first raised drain region 57 is formed directly on each exposedsurface of the first planar drain regions 37. A raised strap region 51is formed directly on each exposed surface of the conductive trench fillregions 60. Due to the lateral growth of the deposited semiconductormaterial, each first raised drain region 57 laterally contacts anadjacent raised strap region 51, thereby providing an electricallyconductive path from a planar source region 33 to a conductive trenchfill region 60. A second raised source region 63 is formed directly oneach exposed surface of the second planar source regions 43. A secondraised drain region 67 is formed directly on each exposed surface of thesecond planar drain regions 47. The first raised source regions 53, thefirst raised drain regions 57, the raised strap regions 51, the secondraised source regions 63, and the second raised drain regions 67 areformed above the SOI substrate 8.

Because the first planar source regions 33, the first planar drainregions 37, the second planar source regions 43, and the second planardrain regions 47 are single crystalline and the selective epitaxypreserves epitaxial alignment between the underlying semiconductormaterial portions and the deposited semiconductor material portions, thefirst raised source regions 53, the first raised drain regions 57, thesecond raised source regions 63, and the second raised drain regions 67are single crystalline. Because the conductive trench fill regions 60are polycrystalline, the raised strap regions 51 are alsopolycrystalline.

Further, because the growth rate of a single crystalline semiconductormaterial is greater than the growth of a polycrystalline semiconductormaterial at the same deposition condition, i.e., temperature, pressure,and reactant supply, the thickness of the first raised source regions53, the first raised drain regions 57, the second raised source regions63, and the second raised drain regions 67 is typically greater than thethickness of the raised strep regions 51. In one embodiment, each of thefirst raised source regions 53, the first raised drain regions 57, thesecond raised source regions 63, and the second raised drain regions 67has substantially the same thickness, which is greater than thethickness of the raised strap regions 51.

In one embodiment, the first raised source regions 53, the first raiseddrain regions 57, the second raised source regions 63, and the secondraised drain regions 67 are deposited as single crystalline dopedsemiconductor regions during the selective epitaxy, and the raised strapregions 51 are deposited as polycrystalline doped semiconductor regionsduring the selective epitaxy.

In another embodiment, a thin dielectric layer (not shown) can beemployed as a masking layer and selective epitaxy may be employed toeffect deposition of a doped semiconductor material only within theeDRAM region E. Alternately, the masking layer may be employed to effectdeposition of the doped semiconductor material within the eDRAM region Eand a subset of the at least one second transistor T2 having secondplanar source and drain regions (43, 47) having a same conductivity typeas the first planar source and drain regions (33, 37).

In still another embodiment, the first raised source regions 53, thefirst raised drain regions 57, the second raised source regions 63, andthe second raised drain regions 67 are deposited as single crystallineundoped semiconductor regions during the selective epitaxy, and theraised strap regions 51 are deposited as polycrystalline undopedsemiconductor regions during the selective epitaxy. The first raisedsource regions 53, the first raised drain regions 57, and the raisedstrap regions 51 are subsequently implanted with dopants of the secondconductivity type. The second raised source regions 63 and the secondraised drain regions 67 are subsequently implanted with dopants of aconductivity type that matches the underlying second planar source ordrain regions (43, 47).

In one embodiment, the planar source regions (33, 43) and the planardrain regions (37, 47) extend from the top surface of the buriedinsulator layer 20 to the topmost surface of the SOI substrate 8. Inthis case, the at least one first transistor T1 and the at least onesecond transistor T2 are fully depleted transistors.

Within each first transistor T1, a first planar source region 33 and afirst raised source region 53 collectively constitute the source regionof the first transistor T1, and a first planar drain region 37 and afirst raised drain region 57 collectively constitute the drain region ofthe first transistor T1. Within each second transistor T2, a secondplanar source region 43 and a second raised source region 63collectively constitute the source region of the second transistor T2,and a second planar drain region 47 and a second raised drain region 67collectively constitute the drain region of the second transistor T2.

Referring to FIG. 7, various metal semiconductor alloy portions areformed on exposed semiconductor surfaces. The first and seconddielectric gate caps (38, 48) can be removed prior to formation of thevarious metal semiconductor alloy portion so that metal semiconductoralloy portions can be formed on the first and second gate conductors(34, 44). The various metal semiconductor alloy portions can be formedby depositing a metal layer (not shown) and inducing a reaction betweenthe metal layer and underlying semiconductor material portions.

In case the semiconductor substrate comprises silicon, the metalsemiconductor alloys are silicides. The metal layer comprises a metalthat can react with silicon to form a metal silicide. For example, themetal can be Ti, Co, Ni, Ta, W, Pt, Pd or an alloy thereof. The typicalthickness of the metal layer ranges from 10 nm to 50 nm, more typicallyfrom 5 nm to 20 nm. Such a metal layer can be readily deposited by anysuitable deposition technique, including, but not limited to: atomiclayer deposition (ALD), chemical vapor deposition (CVD), and physicalvapor deposition (PVD). The metal layer can be deposited alone, ortogether with a metal nitride capping layer (not shown) containing TiNor TaN and having a thickness ranging from 5 nm to 50 nm, and typicallyfrom 10 nm to 20 nm.

After deposition of the metal layer and/or the metal nitride cappinglayer, the entire structure is annealed at a pre-determined elevatedtemperature at which the deposited metal layer reacts with exposedsilicon to form metal silicides of relatively low contact resistance.The annealing is typically performed in an inert gas atmosphere, e.g.,He, Ar, N₂, or forming gas, at relatively low temperatures ranging from100° C. to 600° C., typically from 300° C. to 500° C., and moretypically from 300° C. to 450° C., by using a continuous heating regimeor various ramp and soak heating cycles. Unreacted metal and/or metalnitride are removed after formation of the metal silicides. For certainmetals such as Co, Ti, or Ni, a second annealing step can be employed toform metal silicides of lower resistivity, such as CoSi₂, TiSi₂, orNiSi.

A first source-side metal semiconductor alloy portion 73 is formed on anupper surface of each contiguous pair of a first raised source region 53and a raised strap region 51. A first drain-side metal semiconductoralloy portion 77 is formed on an upper surface of each first raiseddrain region 57. A second source-side metal semiconductor alloy portion83 is formed on an upper surface of each second raised source region 63.A second drain-side metal semiconductor alloy portion 87 is formed on anupper surface of each second raised drain region 67. A first gate-sidemetal semiconductor alloy portion 74 can be formed on each first gateconductor 34, and a second gate-side metal semiconductor alloy portion84 can be formed on each second gate conductor 44.

Referring to FIGS. 8 and 9, a middle-of-line (MOL) dielectric layer 90is deposited the various metal semiconductor alloy portions (73, 77, 74,83, 87, 84). The MOL dielectric layer 90 can comprise, for example, aCVD oxide. The CVD oxide can be an undoped silicate glass (USG),borosilicate glass (BSG), phosphosilicate glass (PSG), fluorosilicateglass (FSG), borophosphosilicate glass (BPSG), or a combination thereof.The thickness of the MOL dielectric layer 82 can be from 200 nm to 500nm. The MOL dielectric layer 90 is typically planarized, for example, bychemical mechanical polishing (CMP).

Various contact via holes are formed in the MOL dielectric layer 90 andfilled with metal to from various contact via structures. Specifically,bit line contact via structures 97 can be formed to contact each of thefirst drain-side metal semiconductor alloy portions 77. Word linecontact via structures 94 can be formed to contact each of the firstgate-side metal semiconductor alloy portions 74. The first source-sidemetal semiconductor alloy portion 73 need not be contacted by aconductive via because a DRAM cell does not require an externalconnection between an access transistor and a capacitor. A source-sidecontact via structure 93 can be formed to contact each of the secondsource-side metal semiconductor alloy portions 83. A drain-side contactvia structure 96 can be formed to contact each of the second drain-sidemetal semiconductor alloy portions 87. A gate-side contact via structure94 can be formed to contact each of the second gate-side metalsemiconductor alloy portions 84.

The thickness of the planar portions, i.e., portions having a constantthickness, of the first raised source regions 53, the first raised drainregions 57, the second raised source regions 63, and the second raiseddrain regions 67 after formation of the various metal semiconductoralloy portions (73, 77, 74, 83, 87, 84) is herein referred to as a firstthickness t1. The thickness of the planar portions of the raised strapregions 51 is herein referred to as a second thickness t2. The firstthickness t1 is greater than the second thickness t2. For example, thefirst thickness t1 can be from 3 nm to 60 nm, and the second thicknesst2 can be from 1.5 nm to 30 nm. Typically, the second thickness t2 isfrom 25% to 75% of the first thickness t1.

Referring to FIG. 10, a second exemplary semiconductor structureaccording to a second embodiment of the present invention is derivedfrom the first exemplary semiconductor structure of FIG. 4.Specifically, the first and second prototype shallow trench isolationstructures (100A′, 100B′) are recessed to form first and second shallowtrench isolation structures (100A, 100B) employing the same methods asin the first embodiment. Subsequently, the pad layer 40 is removed.First and second gate stacks (32, 34, 38, 42, 44, 48) and first andsecond dielectric gate spacers (36, 46) are formed in the same manner asin the first embodiment.

Selective epitaxy of an undoped semiconductor material is subsequentlyperformed. During the selective epitaxy, the undoped semiconductormaterial is deposited only on semiconductor surfaces, but is notdeposited on dielectric surfaces such as the surfaces of the first andsecond dielectric gate caps (38, 48), the first and second dielectricgate spacers (36, 46) and the first and second shallow trench isolationstructures (100A, 100B). A first undoped raised source region 53′ isformed directly on each exposed surface of the first planar sourceregions 33. A first undoped raised drain region 57′ is formed directlyon each exposed surface of the first planar drain regions 37. An undopedraised strap region 51′ is formed directly on each exposed surface ofthe conductive trench fill regions 60. Due to the lateral growth of thedeposited semiconductor material, each first raised drain region 57laterally contacts an adjacent raised strap region 51, thereby providinga physically contiguous adjoined semiconductor material portions above aplanar source region 33 and a conductive trench fill region 60. A secondundoped raised source region 63′ is formed directly on each exposedsurface of the second planar source regions 43. A second undoped raiseddrain region 67′ is formed directly on each exposed surface of thesecond planar drain regions 47. The first undoped raised source regions53′, the first undoped raised drain regions 57′, the undoped raisedstrap regions 51′, the second undoped raised source regions 63′, and thesecond undoped raised drain regions 67′ are formed above the SOIsubstrate 8.

Because the first planar source regions 33, the first planar drainregions 37, the second planar source regions 43, and the second planardrain regions 47 are single crystalline and the selective epitaxypreserves epitaxial alignment between the underlying semiconductormaterial portions and the deposited semiconductor material portions, thefirst undoped raised source regions 53′, the first undoped raised drainregions 57′, the second undoped raised source regions 63′, and thesecond undoped raised drain regions 67′ are single crystalline. Becausethe conductive trench fill regions 60 are polycrystalline, the undopedraised strap regions 51′ are also polycrystalline.

Further, because the growth rate of a single crystalline semiconductormaterial is greater than the growth of a polycrystalline semiconductormaterial at the same deposition condition, i.e., temperature, pressure,and reactant supply, the thickness of the first undoped raised sourceregions 53′, the first undoped raised drain regions 57′, the secondundoped raised source regions 63′, and the second undoped raised drainregions 67′ is typically greater than the thickness of the undopedraised strep regions 51′. In one embodiment, each of the first undopedraised source regions 53′, the first undoped raised drain regions 57′,the second undoped raised source regions 63′, and the second undopedraised drain regions 67′ has substantially the same thickness, which isgreater than the thickness of the undoped raised strap regions 51′.

Dopants are introduced into the first undoped raised source regions 53′,the first undoped raised drain regions 57′, the second undoped raisedsource regions 63′, the second undoped raised drain regions 67′, and theundoped raised strap regions 51′ by ion implantation employing at leastthe first and second gate stacks (32, 34, 38, 42, 44, 48) as animplantation mask. Additional masking layer(s) can be optionallyemployed to limit the area of ion implantation for each round of ionimplantation. For example, a first masking layer can be employed toexpose areas of n-type transistors, while blocking areas for p-typetransistors during implantation of n-type dopants. Subsequently, asecond masking layer can be employed to expose areas of p-typetransistors, while blocking areas for n-type transistors duringimplantation of p-type dopants.

The first undoped raised source regions 53′, the first undoped raiseddrain regions 57′, the undoped raised strap regions 51′, the underlyingportions of the top semiconductor layer 30 beneath the first undopedraised source regions 53′, and the underlying portions of the topsemiconductor layer 30 beneath the first undoped raised drain regions57′ are implanted with dopants of the second conductivity type. Thesecond undoped raised source regions 63′, the second undoped raiseddrain regions 67′, the underlying portions of the top semiconductorlayer 30 beneath the second undoped raised source regions 63′, and theunderlying portions of the top semiconductor layer 30 beneath the secondundoped raised drain regions 67′ are implanted with dopants ofappropriate conductivity type(s). The resulting structure is identicalto the structure shown in FIG. 6 according to the first embodiment.

Referring to FIG. 11, a third exemplary semiconductor structureaccording to the third embodiment of the present invention can bederived from the first exemplary semiconductor structure by setting therecess depth r in FIG. 2 to be lesser than the thickness of the padlayer 40. In this case, the interface between a raised strap region 51and an underlying conductive trench fill region 60 is vertically offsetso that the interface is located at a higher level than the interfacebetween a first raised source region 53 and an underlying first planarsource region 33. In this case, the node dielectric 50 can extend from abottom surface of the deep trench up to the interface between the raisedstrap region 51 and the conductive trench fill region 60.

Referring to FIG. 12, a fourth exemplary semiconductor structureaccording to a fourth embodiment of the present invention can be derivedfrom the first exemplary semiconductor structure by setting the recessdepth r in FIG. 2 to be greater than the thickness of the pad layer 40.In this case, the interface between a raised strap region 51 and anunderlying conductive trench fill region 60 is vertically offset so thatthe interface is located at a lower level than the interface between afirst raised source region 53 and an underlying first planar sourceregion 33. In this case, the node dielectric 50 can extend from a bottomsurface of the deep trench up to the interface between the raised strapregion 51 and the conductive trench fill region 60. A portion of eachsource-side metal semiconductor alloy portion 51, i.e., the portionbelow a dotted line, is located below the topmost surface of the topsemiconductor layer, which includes the first planar source regions 33,the first body regions 35, and the first planar drain regions 37.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the invention is intended to encompassall such alternatives, modifications and variations which fall withinthe scope and spirit of the invention and the following claims.

What is claimed is:
 1. A semiconductor structure comprising: a deeptrench located in a semiconductor-on-insulator (SOI) substrate thatincludes a top semiconductor layer containing a planar source region ofan access transistor; a node dielectric located in said deep trench andcontacting a sidewall of said planar source region; a conductive trenchfill region located in said deep trench; a raised source region locatedabove said top semiconductor layer and contacting an upper surface ofsaid planar source region; and a raised strap region contacting saidraised source region and an upper surface of said conductive trench fillregion.
 2. The semiconductor structure of claim 1, wherein the nodedielectric is contacting a sidewall of said planar source region and asidewall of said conductive trench fill region.
 3. The semiconductorstructure of claim 2, wherein said conductive fill region does notcontact said planar source region, and is laterally spaced from saidplanar source region by said node dielectric.
 4. The semiconductorstructure of claim 2, wherein said node dielectric extends from a bottomsurface of said deep trench to a vertical height above a topmost surfaceof said buried insulator layer.
 5. The semiconductor structure of claim1, wherein an upper portion of said conductive trench fill regioncomprises a doped semiconductor material that contacts said raised strapregion.
 6. The semiconductor structure of claim 1, wherein a body regionof said access transistor comprises a semiconductor material having adoping of a first conductivity type, and each of said planar sourceregion, said raised source region, and said raised strap regioncomprises a semiconductor material having a doping of a secondconductivity type, wherein said second conductivity type is the oppositeof said first conductivity type.
 7. The semiconductor structure of claim1, further comprising a shallow trench isolation structure comprising adielectric material, wherein an outer sidewall of said shallow trenchisolation structure is laterally spaced from said planar source regionby a portion of said node dielectric and an upper portion of saidconductive trench fill region.
 8. The semiconductor structure of claim1, wherein said raised source region and raised strap region comprise adoped semiconductor material.
 9. The semiconductor structure of claim 8,wherein said raised source region is a single crystalline region of saiddoped semiconductor material, and said raised strap region is apolycrystalline region of said doped semiconductor material.
 10. Thesemiconductor structure of claim 8, wherein a planar portion of saidraised source region has a first thickness, and a planar portion of saidraised strap region has a second thickness, wherein said first thicknessis greater than said second thickness.
 11. The semiconductor structureof claim 10, further comprising a raised drain region located on aplanar drain region of said access transistor, wherein a planar portionof said raised drain region has said first thickness.
 12. Thesemiconductor structure of claim 8, further comprising a source-sidemetal semiconductor alloy portion located on an upper surface of saidraised source region and an upper surface of said raised strap region.13. A semiconductor structure comprising: a deep trench located in asemiconductor-on-insulator (SOI) substrate that includes a topsemiconductor layer containing a planar source region of an accesstransistor; a conductive trench fill region located in said deep trench;a raised source region comprising a doped semiconductor material locatedabove said top semiconductor layer and contacting an upper surface ofsaid planar source region; and a raised strap region comprising a dopedsemiconductor material contacting said raised source region and an uppersurface of said conductive trench fill region.
 14. The semiconductorstructure of claim 13, wherein said raised source region is a singlecrystalline region of said doped semiconductor material, and said raisedstrap region is a polycrystalline region of said doped semiconductormaterial.
 15. The semiconductor structure of claim 13, wherein a planarportion of said raised source region has a first thickness, and a planarportion of said raised strap region has a second thickness, wherein saidfirst thickness is greater than said second thickness.
 16. Thesemiconductor structure of claim 15, further comprising a raised drainregion located on a planar drain region of said access transistor,wherein a planar portion of said raised drain region has said firstthickness.
 17. The semiconductor structure of claim 13, furthercomprising a source-side metal semiconductor alloy portion located on anupper surface of said raised source region and an upper surface of saidraised strap region.